In recent years, there have been advances toward high-definition, high-pixel-count solid-state imaging devices, such as CCDs, CMOS sensors, and so forth, and there is also a demand for achieving an increased image-capturing area, that is, a wider viewing angle. On the other hand, there is a high demand for achieving a size reduction of imaging optical systems from the viewpoint of portability and mobility. In particular, with imaging optical systems designed for an endoscope, it is necessary to achieve a size reduction, a cost reduction, and a wider viewing angle at the same time as achieving high definition, and thus, there is a demand for an objective optical system having a wide-angle configuration in which aberrations are also suppressed with a small number of lenses.
Conventional objective optical systems designed for an endoscope generally have a retrofocus-type four- to six-lens configuration (for example, see Patent Literature 1) or a two- to four-lens configuration (for example, see Patent Literatures 2, 3, and 5). In addition, there are known systems that are designed for digital cameras and camera-equipped cellular phones, having a three-lens configuration, referred to as triplet (for example, see Patent Literature 4), or a four-group configuration (for example, see Patent Literature 6).